1. Field of the Invention
The present invention relates to a motor driving device which is mainly formed of a semiconductor integrated circuit and supplies a driving current to drive a three-phase motor.
2. Description of Related Art
Output transistors supplying current to stator coils at high voltage and high electric power are placed at an output stage in a semiconductor integrated circuit so as to drive a three-phase motor. Each of these output transistors is frequently formed of an n-channel metal-oxide semiconductor field effect transistor (nMOSFET, hereinafter called n-channel transistor). The reason is that the on-state resistance per a unit area in a p-channel MOSFET (pMOSFET, hereinafter called p-channel transistor) is larger than that in the n-channel transistor. Therefore, when the output transistors are formed of p-channel transistors respectively, the manufacturing cost of a motor driving device having the output transistors is heightened.
FIG. 3 is a circuit view showing the configuration of a conventional motor driving device. In FIG. 3, 31u, 31v and 31w indicate three stator coils of a three-phase motor connected with each other in a Y shape. 1 indicates a direct voltage source. 2 indicates a high voltage source. 3 indicates the ground. 4 and 5 indicate a pair of n-channel output transistors. A driving current passes through the two stator coils and the n-channel output transistor 4 or 5 to drive the three-phase motor. 6 indicates a p-channel transistor used to drive the n-channel output transistor 4. 7 indicates an n-channel transistor used to drive the n-channel output transistor 4. 8 indicates an inverter for inverting a driving pulse A to input the inverted driving pulse A to gates of the transistors 6 and 7. 9 and 10 indicate two inverters serially connected with each other, and a driving pulse B is inverted twice in the inverters 9 and 10. 21 and 22 indicate two Zener diodes serially connected with each other in opposite directions. A voltage clamp circuit is formed of the Zener diodes 21 and 22.
A source of the p-channel transistor 6 is connected to the high voltage source 2, a drain of the p-channel transistor 6 is connected to a gate of the n-channel output transistor 4, and a gate of the p-channel transistor 6 is connected to an output terminal of the inverter 8. A drain of the n-channel transistor 7 is connected to the gate of the n-channel output transistor 4, a source of the n-channel transistor 7 is connected to the ground 3, and a gate of the n-channel transistor 7 is connected to the output terminal of the inverter 8.
A drain of the n-channel output transistor 4 is connected to the direct voltage source 1, and a source of the n-channel output transistor 4 is connected to a terminal W of the stator coil 31w. A drain of the n-channel output transistor 5 is connected to the terminal W of the stator coil 31w, a source of the n-channel output transistor 5 is connected to the ground 3, and a gate of the n-channel output transistor 5 is connected to an output terminal of the inverter 9. The driving pulse B is inverted in the inverter 10 and the inverter 9 in that order and is applied to the gate of the n-channel output transistor 5.
Also, the voltage clamp circuit composed of the Zener diodes 21 and 22 is placed to connect the gate and the source of the n-channel output transistor 4. The voltage clamp circuit is used to protect the n-channel output transistor 4 from an excess positive voltage and an excess negative voltage applied to the gate of the n-channel output transistor 4.
Also, a motor driving device having the same configuration as that of the motor driving device shown in FIG. 3 is connected to each of terminals U and V of the stator coils 31u and 31v. 
Next, an operation of the motor driving device connected to the terminal W of the stator coil 31w will be described below.
FIG. 4 is a time chart of the driving pulses A and B input to the inverters 8 and 10 respectively, and each of FIG. 5A, FIG. 5B and FIG. 5C is an explanatory view showing an operation of the motor driving device.
As shown in FIG. 4, a timing of inputting the driving pulse A to the inverter 8 differs from a timing of inputting the driving pulse B to the inverter 10. As shown in FIG. 5A, when the driving pulse A is set to a high level, the driving pulse B is set to a low level. In this case, the gates of the transistors 6 and 7 are set to a low level due to the driving pulse A inverted in the inverter 8, the p-channel transistor 6 is turned on, and the n-channel transistor 7 is turned off. Thereafter, the gate of the n-channel output transistor 4 is set to a high level, and the n-channel output transistor 4 is turned on. Also, the gate of the n-channel output transistor 5 is set to a low level due to the driving pulse B inverted twice in the inverters 9 and 10, and the n-channel output transistor 5 is turned off. Therefore, in a first operation, a driving current is supplied from the direct voltage source 1 to the stator coils 31w and 31v through the n-channel output transistor 4 to drive the three-phase motor.
Also, as shown in FIG. 5B, when the driving pulse B is set to a high level, the driving pulse A is set to a low level. In this case, the gates of the transistors 6 and 7 are set to a high level due to the driving pulse A inverted in the inverter 8, the p-channel transistor 6 is turned off, the n-channel transistor 7 is turned on, the gate of the n-channel output transistor 4 is set to a low level, and the n-channel output transistor 4 is turned off. Also, the gate of the n-channel output transistor 5 is set to a high level due to the driving pulse B inverted twice in the inverters 9 and 10, and the n-channel output transistor 5 is turned on. Also, the direct voltage source 1 is connected to the terminal U of the stator coil 31u due to the operation of the motor driving device connected to the terminal U. Therefore, in a second operation, a driving current supplied from the direct voltage source 1 flows through the stator coils 31u and 31w and goes to the ground 3 through the n-channel output transistor 5 to drive the three-phase motor.
In cases where the three-phase motor is driven, the driving pulses A and B are set to the low level together in a stop time period other than the active time period of the output transistor 4 or 5 shown in FIG. 5A or FIG. 5B. In this stop time period, as shown in FIG. 5C, both the n-channel output transistors 4 and 5 are set to the off-state together, no current passes through the stator coil 31w, and the motor driving device is set to a high impedance when the motor driving device placed at the output stage of the semiconductor integrated circuit is seen from the stator coil 31w. In this case, when the n-channel output transistor 4 or 5 is set to the off-state during the stop time period after the first or second operation shown in FIG. 5A or FIG. 5B, charge supplied from the direct voltage source 1 remains in the stator coil 31w, and the terminal W is set to a high voltage due to the remaining charge. Therefore, in cases where a voltage higher than a withstand voltage between the gate and the source of the n-channel output transistor 4 is supplied from the direct voltage source 1 to the stator coil 31w, it is required to protect the n-channel output transistor 4 from the excess voltage applied between the gate and the source of the n-channel output transistor 4 and to prevent the gate of the n-channel output transistor 4 from being damaged.
To protect the n-channel output transistor 4, the voltage clamp circuit composed of the Zener diodes 21 and 22 is placed to connect the gate and the source of the n-channel output transistor 4. In a third operation, as shown in FIG. 5C, the voltage clamp circuit composed of the Zener diodes 21 and 22 is operated in the stop time period, and a clamp current flows from the terminal W to the ground 3 through the n-channel transistor 7 to remove the remaining charge.
However, because the conventional motor driving device has the above-described configuration, following problems occur.
When the motor driving device is set to the high impedance in the stop time period, a voltage of the terminal W is generally equal to half of the voltage of the direct voltage source 1. Therefore, the higher the voltage of the direct voltage source 1, the larger the clamp current. In this case, because the remaining charge is discharged to the ground 3 as the clamp current, loss of an electric power is large. Recently, it has been desired to operate a semiconductor integrated circuit at a low consumed electric power. However, the clamp current inevitably occurs in the motor driving device, it is difficult to drive the three-phase motor at a low consumed electric power by supplying a driving current to the three-phase motor from the motor driving device. In particular, when the motor driving device placed at the output stage of the semiconductor integrated circuit is operated at high voltage and high electric power, the influence of the electric power loss due to the clamp current on the consumed electric power of the three-phase motor is very high.
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional motor driving device, a motor driving device formed of a semiconductor integrated circuit in which an electric power consumed to operate a motor is lowered while lowering a clamp current to a low value.
The object is achieved by the provision of a motor driving device including a first output transistor, a first transistor switch, a second output transistor, a voltage clamp circuit and a constant current circuit. In the voltage clamp circuit, charge of a high voltage, which is generated in a line between the first output transistor and the second output transistor when both the first output transistor and the second output transistor are set to the off-state together, is released from the line as a clamp current. In the constant current circuit, the clamp current of the voltage clamp circuit is controlled to a low value.
Therefore, when the clamp current is generated, the clamp current is controlled to a low value so as to lower an electric power consumed for the driving of the motor. Accordingly, the motor driving device appropriate to the semiconductor integrated circuit can be obtained.